Group III nitride LED with silicon carbide substrate

ABSTRACT

The present invention is a semiconductor structure for light emitting devices that can emit in the red to ultraviolet portion of the electromagnetic spectrum. The semiconductor structure includes a Group III nitride active layer positioned between a first n-type Group III nitride cladding layer and a second n-type Group III nitride cladding layer, the respective bandgaps of the first and second n-type cladding layers being greater than the bandgap of the active layer. The semiconductor structure further includes a p-type Group III nitride layer, which is positioned in the semiconductor structure such that the second n-type cladding layer is between the p-type layer and the active layer. The semiconductor structure is built upon a silicon carbide substrate.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a divisional of application Ser. No. 09/760,635filed Jan. 16, 2001 now U.S. Pat. No 6,800,876, entitled “Group IIINitride LED with Undoped Cladding Layer.”

FIELD OF THE INVENTION

This application incorporates entirely by reference co-pending andcommonly-assigned U.S. Pat. No. 6,534,797, previously referred to asapplication Ser. No. 09/706,057 filed Nov. 3, 2000, for Group IIINitride Light Emitting Devices with Gallium-Free Layers.

The present invention relates to semiconductor structures of lightemitting devices, particularly light emitting-diodes and laser diodesformed from Group III nitrides, which are capable of emitting light inthe red to ultraviolet portions of the electromagnetic spectrum.

BACKGROUND OF THE INVENTION

Photonic semiconductor devices fall into three categories: devices thatconvert electrical energy into optical radiation (e.g., light emittingdiodes and laser diodes); devices that detect optical signals (e.g.,photodetectors); and devices that convert optical radiation intoelectrical energy (e.g., photovoltaic devices and solar cells). Althoughall three kinds of devices have useful applications, the light emittingdiode may be the most commonly recognized because of its application tovarious consumer products and applications.

Light emitting devices (e.g., light emitting diodes and laser diodes),herein referred to as LEDs, are photonic, p-n junction semiconductordevices that convert electrical power into emitted light. Perhaps mostcommonly, LEDs form the light source in the visible portion of theelectromagnetic spectrum for various signals, indicators, gauges, anddisplays used in many consumer products (e.g., audio systems,automobiles, household electronics, and computer systems). LEDs aredesirable as light output devices because of their generally longlifetime, their low power requirements, and their high reliability.

Despite widespread use, LEDs are somewhat functionally constrained,because the color that a given LED can produce is limited by the natureof the semiconductor material used to fabricate the LED. As well knownto those of ordinary skill in this and related arts, the light producedby an LED is referred to as “electroluminescence,” which represents thegeneration of light by an electric current passing through a materialunder an applied voltage. Any particular composition that produceselectroluminescent light tends to do so over a relatively narrow rangeof wavelengths.

The wavelength of light (i.e., its color) that can be emitted by a givenLED material is limited by the physical characteristics of thatmaterial, specifically its bandgap energy. Bandgap energy is the amountof energy that separates a lower-energy valence band and a higher energyconduction band in a semiconductor. The bands are energy states in whichcarriers (i.e., electrons or holes) can reside in accordance withwell-known principles of quantum mechanics. The “bandgap” is a range ofenergies between the conduction and valence bands that are forbidden tothe carriers (i.e., the carriers cannot exist in these energy states).Under certain circumstances, when electrons and holes cross the bandgapand recombine, they will emit energy in the form of light. In otherwords, the frequency of electromagnetic radiation (i.e., the color) thatcan be produced by a given semiconductor material is a function of thatmaterial's bandgap energy.

In this regard, narrower bandgaps produce lower energy, longerwavelength photons. Conversely, wider bandgap materials produce higherenergy, shorter wavelength photons. Blue light has a shorterwavelength—and thus a higher frequency—than most other colors in thevisible spectrum. Consequently, blue light must be produced fromtransitions that are greater in energy than those transitions thatproduce green, yellow, orange, or red light. Producing photons that havewavelengths in the blue or ultraviolet portions of the visible spectrumrequires semiconductor materials that have relatively large bandgaps.

The entire visible spectrum runs from the violet at, or about 390nanometers to the red at about 780 nanometers. In turn, the blue portionof the visible spectrum can be considered to extend between thewavelengths of about 425 and 480 nanometers. The wavelengths of about425 nanometers (near violet) and 480 nanometers (near green) in turnrepresent energy transitions of about 2.9 eV and about 2.6 eV,respectively. Accordingly, only a material with a bandgap of at leastabout 2.6 eV can produce blue light.

Shorter wavelength devices offer a number of advantages in addition tocolor. In particular, when used in optical storage and memory devices,such as CD-ROM optical disks, shorter wavelengths enable such storagedevices to hold significantly more information. For example, an opticaldevice storing information using blue light can hold substantially moreinformation in the same space as one using red light.

The basic mechanisms by which light-emitting diodes operate are wellunderstood in this art and are set forth, for example, by Sze, Physicsof Semiconductor Devices, 2d Edition (1981) at pages 681-703.

The common assignee of the present patent application was the first inthis field to successfully develop commercially viable LEDs that emittedlight in the blue color spectrum and that were available in large,commercial quantities. These LEDs were formed in silicon carbide, awide-bandgap semiconductor material. Examples of such blue LEDs aredescribed in U.S. Pat. Nos. 4,918,497 and 5,027,168 to Edmond eachtitled “Blue Light Emitting Diode Formed in Silicon Carbide.” Otherexamples of Group III nitride LED structures and laser structures aredescribed in commonly assigned U.S. Pat. Nos. 5,523,589; 5,592,501; and5,739, 554.

In addition to silicon carbide, candidate materials for blue lightemitting devices are gallium nitride (GaN) and its associated Group III(i.e., Group III of the periodic table) nitride compounds such asaluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), andaluminum indium gallium nitride (AlInGaN). These materials areparticularly attractive because they offer direct energy transitionswith bandgaps between about 1.9 to about 6.2 eV at room temperature.More common semiconductor materials such as silicon, gallium phosphide,or gallium arsenide are unsuitable for producing blue light becausetheir bandgaps are approximately 2.26 eV or less, and in the case ofsilicon, are indirect semiconductors and inefficient light emitters.

As known to those familiar with LEDs and electronic transitions, adirect transition occurs in a semiconductor when the valence band maximaand the conduction band minima have the same momentum state. This meansthat crystal momentum is readily conserved during recombination ofelectrons and holes so that the energy produced by the transition can gopredominantly and efficiently into the photon, (i.e., to produce lightrather than heat). When the conduction band minimum and valence bandmaximum do not have the same momentum state, a phonon (i.e., a quantumof vibrational energy) is required to conserve crystal momentum and thetransition is called “indirect.” The necessity of a third particle, thephonon, makes indirect radiative transitions less likely, therebyreducing the light emitting efficiency of the device.

Generally speaking, an LED formed in a direct bandgap material willperform more efficiently than one formed in an indirect bandgapmaterial. Therefore, the direct transition characteristics of Group IIInitrides offer the potential for brighter and more efficientemissions—and thus brighter and more efficient LEDs—than do theemissions from indirect materials such as silicon carbide. Accordingly,much interest in the last decade has also focused on producing lightemitting diodes in gallium nitride and related Group III nitrides.

Although Group III nitrides offer a direct transition over a widebandgap energy range, the material presents a particular set oftechnical manufacturing problems. In particular, no commercially-viabletechnique has yet emerged for producing bulk single crystals of galliumnitride (GaN) that are capable of functioning as appropriate substratesfor the gallium nitride epitaxial layers on which photonic devices wouldbe formed.

All semiconductor devices require some kind of structural substrate.Typically, a substrate formed of the same material as the active regionoffers significant advantages, particularly in crystal growth andlattice matching. Because gallium nitride has yet to be formed in suchbulk crystals, gallium nitride photonic devices must be formed inepitaxial layers on non-GaN substrates.

Recent work in the field of Group III nitride substrates includescopending and commonly assigned U.S. patent application Ser. No.09/361,945, filed Jul. 27, 1999, for “Growth of Bulk Single Crystals ofAluminum Nitride;” Ser. No. 09/361,944 filed Jul. 27, 1999, for “Growthof Bulk Single Crystals of Aluminum Nitride from a Melt;” Ser. No.09/111,413 filed Jul. 7, 1999, for “Growth of Bulk Single Crystals ofAluminum Nitride;” Ser. No. 09/169,385 filed Oct. 9, 1998, for “Growthof Bulk Single Crystals of Aluminum Nitride: Silicon Carbide Alloys;”and Ser. No. 09/154,363 filed Sep. 16, 1998 for “Vertical Geometry InGaN LED.” All of these pending applications are incorporated entirelyherein by reference.

Using different substrates, however, causes an additional set ofproblems, mostly in the area of crystal lattice matching. In nearly allcases, different materials have different crystal lattice parameters. Asa result, when a gallium nitride epitaxial layer is grown on a differentsubstrate, some crystal lattice mismatching and thermal expansioncoefficient mismatching will occur. The resulting epitaxial layer isreferred to as being “strained” by this mismatch. Crystal latticemismatches, and the strain they produce, introduce the potential forcrystal defects. This, in turn, affects the electronic characteristicsof the crystals and the junctions, and thus tends to degrade theperformance of the photonic device. These kinds of defects are even moreproblematic in high power structures.

In early Group III nitride LEDs, the most common substrate for galliumnitride devices was sapphire (i.e., aluminum oxide Al₂O₃). Certaincontemporary Group III nitride devices continue to use it.

Sapphire is optically transparent in the visible and ultraviolet ranges,but has a crystal lattice mismatch with gallium nitride of about 16percent. Furthermore, sapphire is insulating rather than conductive, andis unsuitable for conductivity doping. Consequently, the electriccurrent that must be passed through an LED to generate the lightemission cannot be directed through a sapphire substrate. Thus, othertypes of connections to the LED must be made.

In general, LEDs with vertical geometry use conductive substrates sothat ohmic contacts can be placed at opposite ends of the device. Suchvertical LEDs are preferred for a number of reasons, including theireasier manufacture and simpler incorporation into end-use devices thannon-vertical devices. In the absence of a conductive substrate, however,vertical devices cannot be formed.

In contrast with sapphire, Gallium nitride only has a lattice mismatchof about 2.4 percent with aluminum nitride (AlN) and mismatch of about3.5 percent with silicon carbide. Silicon carbide has a somewhat lessermismatch of only about 1 percent with aluminum nitride.

Group III ternary and quaternary nitrides (e.g., indium gallium nitrideand aluminum indium gallium nitride) have also been shown to haverelatively wide bandgaps. Accordingly, such Group III nitride solidsolutions also offer the potential for blue and ultravioletsemiconductor lasers and LEDs. These compounds, however, present thesame problems as gallium nitride, namely, the lack of an identicalsingle crystal substrate. Thus, each is typically used in the form ofepitaxial layers grown on different substrates. This presents the samepotential for crystal defects and associated electronic problems.

Accordingly, the assignee of the present invention has developed the useof silicon carbide substrates for gallium nitride and other Group IIIdevices as a means of solving the conductivity problems of sapphire as asubstrate. Because silicon carbide can be doped conductively, verticalLEDs can be formed. As noted, a vertical structure facilitates both themanufacture of LEDs and their incorporation into circuits and end-usedevices.

As known to those familiar with Group III nitrides, their propertiesdiffer based on the identity and mole fraction of the present Group IIIelements (e.g., gallium, aluminum, indium). For example, increasing themole fraction of aluminum tends to increase the bandgap, whiledecreasing the amount of aluminum tends to increase the refractiveindex. Similarly, a larger proportion of indium will decrease thebandgap of the material, thus permitting the bandgap to be adjusted or“tuned” to produce photons of desired frequencies. Changing the molarproportions in the solutions also changes the crystal lattice spacing.Accordingly, and despite much effort in this area, a need still existsfor devices that incorporate vertical geometry, and that take advantageof the characteristics that result when the proportions of indium,aluminum, and gallium are desirably adjusted in the active layers,cladding layers, and buffer layers of Group III nitride photonicdevices.

It is a further object of the present invention to provide lightemitting devices having decreased nonradiative recombination andimproved efficiency.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to produce lightemitting diodes and laser diodes from Group III nitrides in a mannerthat takes advantage of their favorable properties.

The invention meets these objects via a semiconductor structure thatincludes a Group III nitride active layer positioned between a firstn-type Group III nitride cladding layer and a second n-type Group IIInitride cladding layer. The active layer has a bandgap that is smallerthan the respective bandgaps of the first and second n-type claddinglayers. The semiconductor structure further includes a p-type Group IIInitride layer, which is positioned in the semiconductor structure suchthat the second n-type cladding layer is between the p-type layer andthe active layer.

In another embodiment, the second n-type Group III nitride claddinglayer has a graded composition such that it is substantially latticematched to the p-type Group III nitride layer at the junction with thep-type Group III nitride layer.

In another embodiment, the structure includes a third n-type Group IIInitride layer positioned between the second n-type Group III nitrideclad layer and the p-type Group III nitride layer. The third n-typeGroup III nitride layer preferably comprises the same material structureas the p-type Group III nitride layer such that a p-n homojunction isformed between the third n-type Group III nitride layer and the p-typeGroup III nitride layer.

The foregoing, as well as other objectives and advantages of theinvention and the manner in which the same are accomplished, is furtherspecified within the following detailed description and its accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional schematic view of the aspects of asemiconductor structure for a light emitting device according to thepresent invention;

FIG. 2 is a plot of bandgap energy versus lattice parameter for GroupIII nitrides alloys of aluminum, indium, and gallium (assuming a linearinterpolation);

FIG. 3 is a cross-sectional schematic view of an embodiment of thesemiconductor structure;

FIG. 4 is a cross-sectional schematic view of an embodiment of thesemiconductor structure;

FIG. 5 is a cross-sectional schematic view of an embodiment of thesemiconductor structure;

FIG. 6 is a cross-sectional schematic view of an embodiment of thesemiconductor structure;

FIG. 7 is a cross-sectional schematic view of an embodiment of thesemiconductor structure;

FIGS. 8 and 9 are bandgap diagrams corresponding to certain prior artdevices; and

FIGS. 10-12 are bandgap diagrams for devices according to the presentinvention.

DETAILED DESCRIPTION

The present invention is a semiconductor structure for light emittingdevices that can emit in the red to ultraviolet portion of theelectromagnetic spectrum. The structure includes a Group III nitrideactive layer positioned between a first n-type Group III nitridecladding layer and a second n-type Group III nitride cladding layer. Thesecond n-type cladding layer is characterized by the substantial absenceof magnesium (i.e., magnesium may be present, but only in amounts thatare so small as to have no functional effect on the semiconductordevice). The semiconductor structure itself is further characterized bya p-type Group III nitride layer, which is positioned in thesemiconductor structure such that the second n-type cladding layer isbetween the p-type layer and the active layer. In addition, the activelayer has a bandgap that is smaller than each respective bandgap of thefirst and second n-type cladding layers. As used herein, the term“layer” generally refers to a single crystal epitaxial layer.

A particular conductivity type (i.e., n-type or p-type) may beunintentional, but is more commonly a result of specifically doping theGroup III nitrides using the appropriate donor or acceptor atoms. It isdesirable to include layers of opposite conductivity types in order toform a p-n junction in the device. Under forward voltage bias, minoritycarriers injected across the p-n junction recombine to produce thedesired luminescent emissions. Appropriate doping of Group III nitridesis well understood in the art and will not be further discussed hereinother than as necessary to describe the invention.

In general, the active layer and the cladding layers comprise GroupIII-nitride compounds. The Group III elements in such compounds may bealuminum, indium, gallium, or a combination of two or more suchelements.

As will be understood by those having ordinary skill in the art, themolar fraction of aluminum, indium, and gallium in the active layer, thefirst n-type cladding layer, and the second n-type cladding layer may begenerally expressed by the formula, Al_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1and 0≦y<1 and (x+y)≦1. In this regard, the relative concentrations ofaluminum, indium, and gallium may vary from layer to layer. It will beunderstood by those skilled in the art, however, that a cladding layercannot be indium nitride (i.e., y=1) because InN has the lowest bandgapof all possible combinations and the active layer cannot be aluminumnitride(i.e., x=1) because AlN has the highest bandgap of all possiblecombinations. It will be understood in these embodiments that thecladding layers will have a larger energy band gap than the activelayer.

An understanding of the invention may be achieved with reference to FIG.1, which is a cross-sectional schematic view of a semiconductorstructure for an LED according to the present invention. Thesemiconductor structure, which is generally designated at 10, includes afirst n-type cladding layer 11 of Al_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1and 0≦y<1 and (x+y)≦1.

The semiconductor structure 10 also includes a second n-type claddinglayer of Al_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1 and 0≦y<1 and (x+y)≦1, orin a more specific embodiment, an indium-free aluminum gallium nitriden-type cladding layer 12 having the formula, Al_(x)Ga_(1-x)N, where0<x<1. In this regard, the range for the variable x excludes both 0 and1, which will be understood by those skilled in the art as requiring thepresence of both aluminum and gallium (i.e., an alloy of aluminum andgallium). As noted, the second n-type cladding layer 12 specificallyexcludes magnesium, and may be doped or undoped. The cladding layers maybe unintentionally n-type, i.e. undoped.

An n-type active layer 13 having the formula Al_(x)In_(y)Ga_(1-x-y)N,where 0≦x<1 and 0≦y≦1 and (x+y)≦1, is positioned between the firstn-type cladding layer and the second n-type cladding layer 12. In a morespecific embodiment, the active layer 13 is aluminum-free, consistingessentially of an indium gallium nitride having the formula,In_(y)Ga_(1-y)N, where 0<y<1. In this regard, the range for the variabley excludes both 0 and 1, which will be understood by those skilled inthe art as requiring the presence of both indium and gallium (i.e., analloy of indium and gallium).

The semiconductor structure is further characterized by a p-type GroupIII nitride layer 18, which as previously noted, is positioned in thesemiconductor structure such that the second n-type cladding layer 12 isbetween the p-type layer 18 and the active layer 13. In preferredembodiments, the p-type layer is made of gallium nitride (preferablymagnesium-doped gallium nitride); indium nitride; or indium galliumnitride of the formula In_(x)Ga_(1-x)N, where 0<x<1.

Note that in embodiments wherein the p-type layer 18 is made ofmagnesium-doped gallium nitride, the second n-type cladding layer 12should be thick enough to deter migration of magnesium from the p-typelayer 18 to the active layer 13, yet thin enough to facilitaterecombination of electrons and holes in the active layer 13. This helpsto maximize emissions from the active layer 13. Moreover, because thep-n junction is not formed at the interface between an InGaN layer andan AlGaN layer—i.e. an InGaN/AlGaN p-n junction is avoided the interfaceshould have a reduced density of interface states. Such a reduction ininterface states should result in more efficient recombination ofcarriers in the active layer, with a corresponding increase in overalldevice efficiency.

In another embodiment, the p-type layer comprises a p-type superlatticeformed of selectively doped p-type Group III nitride layers selectedfrom the group consisting of gallium nitride; indium nitride; and indiumgallium nitride of the formula In_(x)Ga_(1-x)N, where 0<x<1. Inparticular, the superlattice is best formed from alternating layers ofany two of these Group III nitride layers. In such a superlattice,alternating layers of gallium nitride and indium gallium nitride aremost preferred.

The active layer 13 may be doped or undoped. As is known to thosefamiliar with Group III nitride properties, the undoped material willgenerally be unintentionally n-type, and that is the case for the secondn-type cladding layer 12. In particular, the first n-type cladding layer11 and the second n-type cladding layer 12 have respective bandgaps thatare each larger than the bandgap of the active layer 13.

The Group III mole fractions can be selected to provide thesecharacteristics. For example, FIG. 2 theoretically describes bandgapenergy versus lattice parameter. The triangular region of FIG. 2represents the range of bandgap energies available for Group IIInitrides of aluminum, indium, and gallium. FIG. 2 reveals that for anyparticular lattice parameter, eliminating gallium maximizes the bandgapenergy (i.e., the bandgap for an aluminum indium nitride is defined bythe AlN-InN segment).

As is known to those familiar with semiconductor structures-especiallylaser structures, the active layer must have a lower bandgap than theadjacent n-type cladding layers, and a higher refractive index than theadjacent cladding layers. Such a structure gives two benefits importantfor laser capability. First, if the active layer has the lowest bandgap,it may form a quantum well into which carriers tend to fall. This helpsto enhance the device efficiency. Second, waveguiding occurs in thematerial that has the highest refractive index in the structure.Accordingly, when the bandgap of the active layer is less than that ofthe adjacent layers and its refractive index is greater than that of theadjacent layers, the lasing capabilities of the device are enhanced.

Moreover, as known to those of ordinary skill in this art, thecomposition of ternary and quaternary Group III nitrides can affect boththeir refractive index and their, bandgap. Generally speaking, a largerproportion of aluminum increases the bandgap and decreases therefractive index. Thus, in preferred embodiments, in order for thecladding layers 11 and 12 to have a bandgap larger than the active layer13 and a refractive index smaller than the active layer 13, the claddinglayers 11 and 12 preferably have a higher fraction of aluminum orgallium as compared to the active layer 13. The larger bandgap of thecladding layers 11 and 12 encourages carriers to be confined in theactive layer 13, thereby increasing the efficiency of the device.Similarly, the lower refractive index of the heterostructure layers 11and 12 encourages the light to be more preferably guided along (i.e.,confined to) the active layer 13.

As previously noted, the recited variables (e.g., x and y) refer to thestructural layer they describe. That is, the value of a variable withrespect to one layer is immaterial to the value of the variable withrespect to another layer. For example, in describing the semiconductorstructure, the variable x may have one value with respect to firstn-type cladding layer 11, another value with respect to second n-typecladding layer 12, and yet another value with respect to activedescribed layer 13. As will also be understood by those of ordinaryskill in the art, the limitation 0≦(x+y)≦1 in the expressionAl_(x)In_(y)Ga_(1-x-y)N simply requires that the Group III elements andthe nitride be present in a 1:1 molar ratio.

In preferred embodiments, active layer 13 comprises an InGaN layerhaving a mole fraction of indium between about 0.05 and 0.55. Referringto FIGS. 1 and 3, layer 12 is preferably an Al_(x)Ga_(1-x)N layer havinga mole fraction of aluminum between about 0.14 and 0.24, while layer 11is preferably an Al_(x)Ga_(1-x)N layer having a mole fraction ofaluminum between about 0 and 0.15. Referring to FIG. 3, layer 19 ispreferably an Al_(x)Ga_(1-x)N layer having a mole fraction of aluminumbetween about 0 and 0.15.

It will be appreciated by those of ordinary skill in the art that, asused herein, the concept of one layer being “between” two other layersdoes not necessarily imply that the three layers are contiguous (i.e.,in intimate contact). Rather, as used herein the concept of one layerbeing between two other layers is meant to describe the relativepositions of the layers within the semiconductor structure. Similarly,as used herein, the concept of a first layer being in contact with asecond layer, “opposite” a third layer, merely describes the relativepositions of the first and second layers within the semiconductorstructure.

That said, in preferred embodiments of the semiconductor structure, theactive layer 13 has a first surface 14 contiguous to the first n-typecladding layer 11 and a second surface 15 contiguous to the secondn-type cladding layer 12. In other words, in such embodiments, theactive layer 13 is sandwiched directly between the first n-type claddinglayer 11 and the second n-type cladding layer 12, with no additionallayers disturbing this three-layer isotype heterostructure (i.e. aheterostructure in which all of the materials have the same conductivitytype), which is designated by the bracket 16. In another preferredembodiment, the p-type layer 18 is in contact with said second n-typecladding layer 12, opposite said active layer 13.

The structural designation “heterostructure” is used in a manner wellunderstood in this art. Aspects of these structures are discussed, forexample, in Sze, Physics of Semiconductor Devices, Second Edition (1981)at pages 708-710. Although the cited Sze discussion refers to lasers, itnonetheless illustrates the nature of, and the distinction between,homostructure, single heterostructure, and double heterostructuredevices. Isotype heterostructures are discussed by Hartman et al. inU.S. Pat. No. 4,313,125, which is hereby incorporated herein in itsentirety.

The semiconductor device may also include additional n-type layers ofAl_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1 and 0≦y<1 and (x+y)≦1. In oneembodiment depicted in FIG. 3, a third n-type layer 19 is positionedbetween second n-type cladding layer 12 and p-type layer 18. Preferably,the third n-type layer 19 has a first surface that is in contact withthe p-type layer 18 and a second surface that is in contact with secondn-type cladding layer 12.

Third n-type layer 19 is lattice matched with p-type layer 18.Preferably, third n-type layer 19 forms a p-n homojunction with p-typelayer 18. Having a p-n homojunction reduces the number of interfacestates at the junction. Because such states may result in nonradiativerecombination, reducing the number of such states improves therecombination efficiency, thus improving overall device efficiency.

The semiconductor device 10 can further comprise a silicon carbidesubstrate 17 that has the same conductivity type as first n-typecladding layer 11 (i.e., an n-type silicon carbide substrate). Thesilicon carbide substrate 17 preferably has a polytype of 3C, 4H, 6H, or15R. The first n-type cladding layer 11 is positioned between thesilicon carbide substrate 17 and the active layer 13. In one embodimentof the invention, the silicon carbide substrate 17 is in contact withthe first n-type cladding layer 11, opposite the active layer 13 (i.e.,there are no intervening layers between silicon carbide substrate 17 andfirst n-type cladding layer 11).

The silicon carbide substrate 17 is most preferably a single crystal. Asis well understood by those of ordinary skill in this art, a highquality single crystal substrate provides a number of structuraladvantages that in turn provide significant performance and lifetimeadvantages. The silicon carbide substrate 17 can be formed by themethods described in U.S. Pat. No. 4,866,005 (now U.S. Pat. No. RE34,861). Preferably, the silicon carbide substrate 17 and the firstcladding layer 11 are n-type.

In a preferred embodiment depicted by FIG. 4, the first n-type claddinglayer 11 has a first surface 21 that is in contact with the siliconcarbide substrate 17 and a second surface 22 that is in contact with theactive layer 13. In particular, the composition of the first n-typecladding layer 11 is progressively graded such that the crystal latticeat its first surface 21 more closely matches the crystal lattice of thesilicon carbide 17, and the crystal lattice at its second surface 22more closely matches the crystal lattice of the active layer 13. Asufficient mole fraction of indium should be present in the first n-typecladding layer 11 to ensure that it remains conductive at its firstsurface 21, adjacent to the silicon carbide substrate 17.

As will be understood by those of ordinary skill in the art,progressively grading embraces both step grading and linear grading.Accordingly, as used herein, the concept of more closely matchingrespective crystal lattices does not imply perfect matching, but ratherthat a layer whose composition has been progressively, compositionallygraded so that its lattice at a layer interface is more compatible withthe crystal lattice of the adjacent layer. When fabricating devices, anumber of considerations must be balanced, one of which is latticematching. If other factors are more important, a perfect or closelattice match may be less important, and vice versa.

In this regard, n-type cladding layers, especially aluminum indiumnitride n-type cladding layers, can be selectively lattice matched togallium-containing active layers, especially gallium nitride and indiumgallium nitride active layers, in order to reduce strain and defects. Inparticular, aluminum indium nitrides are useful because they can belattice matched to other Group III nitrides with lower bandgaps andtherefore are useful as cladding layer materials. See FIG. 2.

As will be understood by those having ordinary skill in the art, latticematching of the cladding layers and the active layer can be a one-sidedlattice match (i.e., where a lattice match occurs on one side of theactive layer) or a two-sided lattice match (i.e., where a lattice matchoccurs on both sides of the active layer).

In another embodiment depicted by FIG. 5, the semiconductor structurefurther includes a conductive buffer layer 23 positioned between thesilicon carbide substrate 17 and the first n-type cladding layer 11. Ina variant of this embodiment, the conductive buffer layer 23 issandwiched between the silicon carbide substrate 17 and the first n-typecladding layer 11, with no intervening layers. The conductive bufferlayer 23 preferably consists essentially of aluminum gallium nitridehaving the formula Al_(x)Ga_(1-x)N, where 0<x<1. Alternatively, when thefirst n-type cladding layer 11 consists essentially of aluminum indiumnitride having the formula, Al_(x)In_(1-x)N, where 0<x<1, the conductivebuffer layer 23 preferably consists essentially of aluminum indiumnitride having the formula, Al_(x)In_(1-x)N, where 0<x<1. Otheracceptable buffers and buffer structures include those described incommonly assigned U.S. Pat. Nos. 5,523,589, 5,393,993, and 5,592,501,the contents of each hereby being incorporated entirely herein byreference.

To facilitate the transition between the first n-type cladding layer 11and the conductive buffer layer 23, the semiconductor structure canfurther include a Group III nitride transition layer 24, preferablyformed of gallium nitride, that is positioned between the conductivebuffer layer 23 and the first n-type cladding layer 11. See FIG. 6. Thetransition layer 24 has the same conductivity type as the first n-typecladding layer 11 (i.e., an n-type transition layer).

Alternatively, as depicted by FIG. 7, the conductive buffer layer 23 andtransition layer 24 can be replaced by discrete crystal portions 28 thatare disclosed more fully in commonly assigned U.S. patent applicationSer. No. 08/944,547, filed Oct. 7, 1997, now U.S. Pat. No. 6,201,262 for“Group III Nitride Photonic Devices on Silicon Carbide Substrates withConductive Buffer Interlayer Structure,” which is incorporated entirelyherein by reference.

In yet another embodiment, the semiconductor structure 10 furtherincludes a first ohmic contact 25 and a second ohmic contact 26. Asindicated in FIG. 1, the first ohmic contact 25 is positioned in thesemiconductor structure such that the silicon carbide substrate 17 isbetween the first ohmic contact 25 and the first n-type cladding layer11. The second ohmic contact 26 is positioned in the semiconductorstructure such that the p-type layer 18 is between the second ohmiccontact 26 and the second n-type cladding layer 12.

Preferably, the first ohmic contact 25 is placed directly on the siliconcarbide substrate 17, opposite the first n-type cladding layer 11 (oropposite the conductive buffer layer 23 or discrete crystal portions 28,depending on the particular structural embodiment), and the second ohmiccontact 26 is placed directly on the p-type layer 18, opposite thesecond n-type cladding layer 12. In a variant of this embodiment, thep-type layer 18 is sandwiched between the second ohmic contact 26 and asecond p-type layer (not shown).

As recognized by those of ordinary skill in this art, the conductivebuffer layer 23 provides a physical and electronic transition betweenthe silicon carbide substrate 17 and the first n-type cladding layer 11.In many circumstances, the presence of the conductive buffer layer 23helps ease the physical strain that can result from the latticedifferences between the silicon carbide substrate 17 and the firstn-type cladding layer 11. Furthermore, to preserve the vertical functionof the device, the conductive buffer layer 23 has to be sufficientlyconductive to carry the desired or required current to operate thesemiconductor device 10. Likewise, the transition layer 24 serves asimilar physical and electronic transition.

The ohmic contacts 25 and 26, which complete the advantageous verticalstructure of the invention, are preferably formed of a metal such asaluminum (Al), nickel (Ni), titanium (Ti), gold (Au), platinum (Pt),vanadium (V), alloys, or blends thereof, or sequential layers of two ormore of these metals, but also may be formed of other ohmic contactmaterials known by those skilled in the art provided that they exhibitohmic character and do not otherwise interfere with the structure orfunction of the light-emitting device 10.

To the extent that the first ohmic contact 25 is formed to the siliconcarbide substrate 17, the invention is distinguished from those devicesthat employ sapphire. Sapphire cannot be made conductive, and so cannotbe connected to an ohmic contact. Consequently, sapphire-based devicescannot be formed into the kinds of vertical structures that are mostpreferred for LEDs.

Accordingly, in one preferred embodiment the invention is asemiconductor structure for light emitting devices that includes ann-type single crystal silicon carbide substrate 17 of a 3C, 4H, 6H, or15R polytype; a p-type layer 18 formed of at least one Group III nitrideselected from the group consisting of gallium nitride (preferablymagnesium-doped gallium nitride), indium nitride, and indium galliumnitride having the formula In_(x)Ga_(1-x)N, where 0<x<1; an undopedactive layer of Al_(x)In_(y)Ga_(1-x-y)N, where 0≦x<1 and 0≦y≦1 and(x+y)≦1; a first n-type cladding layer 11 of Al_(x)In_(y)Ga_(1-x-y)N,where 0≦x≦1 and 0≦y<1 and (x+y)≦1; and a second n-type cladding layer 12of Al_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1 and 0≦y<1 and (x+y)≦1. Mostpreferably, the p-type layer 18 comprises a superlattice formed fromalternating layers of any two of the aforementioned Group III nitrides.

As disclosed previously, the first n-type cladding layer 11 and thesecond n-type cladding layer 12 have respective bandgaps that are eachlarger than the bandgap of the active layer 13. Moreover, the firstn-type cladding layer 11 is positioned between the silicon carbidesubstrate 17 and the active layer 13, the second n-type cladding layer12 is positioned between the active layer 13 and the p-type layer 18,and the active layer 13 is positioned between the first n-type claddinglayer 11 and the second n-type cladding layer 12.

The composition of the first n-type cladding layer 11 can beprogressively graded such that the crystal lattice at its first surface21 more closely matches the crystal lattice of the silicon carbide 17,and the crystal lattice at its second surface 22 more closely matchesthe crystal lattice of the active layer 13. Similarly, the compositionof the second n-type cladding layer 12 can be progressively graded suchthat the crystal lattice at its second surface more closely matches thecrystal lattice of the p-type layer 18. As previously noted,progressively grading across an epitaxial layer embraces both grading insteps and grading continuously (i.e., without steps). Causing the n-typecladding 12 to be substantially lattice matched to the p-type layer 18reduces the number of interface states at the p-n junction formedbetween the layers. Because such states may result in nonradiativerecombination, reducing the number of such states improves therecombination efficiency, thus improving overall device efficiency inthe active layer 13.

Furthermore, and in accordance with the previous descriptions, thispreferred structure may also include one or more of the followinglayers-a third n-type cladding layer 19, the conductive buffer layer 23,the Group III nitride transition layer 24, the discrete crystal portions28, and the ohmic contacts 25 and 26. In this regard, the conductivebuffer layer 23 most preferably is aluminum gallium nitride having theformula Al_(x)Ga_(1-x)N, where 0≦x≦1.

FIGS. 8, 9, 10, 11 and 12 are bandgap diagrams of various structures,including embodiments of the present invention. All of the bandgapdiagrams 8 through 12 represent the bandgaps under forward bias (i.e.“flat-band” conditions). It will be understood by a skilled person thatthe bandgap diagrams 8 through 12 are schematic in nature and are notnecessarily drawn to scale. While they illustrate important aspects ofthe invention, it will be understood that the actual band structure mayvary slightly from the illustration. In FIGS. 8-12, whenever possible,identical numerical designations will refer to identical portions of thediagrams.

FIG. 8 is a bandgap diagram of a prior art device showing an n-typegallium nitride clad layer 30, an indium gallium nitride active layer31, and a p-type aluminum gallium nitride layer 32. In this device, thep-n junction is represented by the dotted line at 33.

With respect to the physical structure of the device and the interfacequality between layers, interfaces between identical materials are theeasiest to make of high quality. Among the Group III nitrides, theinterface between gallium nitride and gallium nitride is the easiest tomake of high quality, with the interface between gallium nitride andaluminum gallium nitride being more difficult, but easier than mostothers. The next-to-worst is the interface between gallium nitride andindium gallium nitride, with the worst interface quality being typicallydemonstrated between indium gallium nitride and aluminum galliumnitride.

Furthermore, it will be recalled that the disassociation temperature ofindium gallium nitride is generally less than all of the other Group IIInitrides. Accordingly, once the InGaN active layer has been grown, thegrowth temperatures for the remaining layers must be limited totemperatures that avoid undesired disassociation or degradation of theindium gallium nitride layer. Stated differently, if the InGaN activelayer were absent, the AlGaN and GaN layers could be grown at highertemperatures that are more favorable (all other factors being equal) forhigher quality epitaxial layers of these materials.

As a result, at the lower growth temperatures used to grow the aluminumgallium nitride layers that are required to protect the indium galliumnitride layer, the resulting quality of the aluminum gallium nitridelayers is somewhat less than it would be if the layers could be grown ata higher temperature.

Accordingly, although ordinarily an AlGaN—AlGaN interface would beconsidered to make a good homojunction, under the lower growthtemperatures required to protect the desired indium gallium nitrideactive layer of the present invention, the aluminum gallium nitridelayers are of poor quality, with the p-type aluminum gallium nitridelayers being particularly bad. As a result, for devices that incorporateindium gallium nitride active layers, interfaces and junctions betweenp-type aluminum gallium nitride and n-type aluminum gallium nitride, aregenerally of very low quality. Thus the invention's avoidance of suchjunctions is counterintuitive and produces an unexpectedly betterdevice. Stated differently, prior art devices that incorporate thestructure of FIG. 8 require interfaces between Group III nitrides thatare difficult to form with high quality.

FIG. 9 illustrates a device described in copending and commonly assignedapplication Ser. No. 09/154,363 filed Sep. 16, 1998. As in FIG. 8, then-type gallium nitride layer is designated at 30, the indium galliumnitride active layer is at 31, the p-n junction is at 33, and the p-typealuminum gallium nitride is designated at 32. The device illustrated inFIG. 9, however, also includes an additional n-type gallium nitride cladlayer 34 that provides a slightly better interface with the indiumgallium nitride active layer 31; i.e. the adjacent GaN-InGaN layer tendsto provide the opportunity for a higher quality interface than doadjacent AlGaN-InGaN layers. FIG. 9 also illustrates an n-type aluminumgallium nitride layer 35 between the second gallium nitride layer 34 andthe p-type aluminum gallium nitride layer 32. Finally, FIG. 9 includesan additional p-type gallium nitride layer 36 as a top contact layer.This device offers the advantage of having the p-n junction 33 formedbetween adjacent layers of aluminum gallium nitride, and the GaN layer34 likewise provides a slightly better interface with the indium galliumnitride active layer 31 than does the AlGaN layer 32 of FIG. 8.

FIG. 10 illustrates the bandgap relationships of the embodiment of thepresent invention as illustrated in FIG. 1 in which the n-type galliumnitride layer 30 (11 in FIG. 1) is again a clad layer for the indiumgallium nitride active layer 31 (13 in FIG. 1). The opposing clad layer36 is formed of n-type aluminum gallium nitride, and the device iscompleted with the p-type gallium nitride layer 36, thus defining thep-n junction 33 between the n-type AlGaN layer 35 and the p-type galliumnitride layer 36. This offers the advantage of having the p-n junctionat the interface between the n-type aluminum gallium nitride 35 and thep-type gallium nitride 36. As noted above, other than an GaN—GaNjunction, the AlGaN-GaN junction is the one most easily formed at thequality required for successful devices.

FIG. 11 illustrates another embodiment of the present invention in whichthe first clad layer is the n-type gallium nitride layer 30, the activelayer is indium gallium nitride 31, and the second clad layer is n-typealuminum gallium nitride 35. This embodiment, however, includes anadditional layer of n-type gallium nitride 37 adjacent the n-typealuminum gallium nitride layer 35. As a result the p-n junction isformed between n-type gallium nitride 37 and p-type gallium nitride 36giving a GaN—GaN interface that provides the highest quality from astructural standpoint.

FIG. 12 illustrates another preferred embodiment in which the n-typegallium nitride layer 30 again forms one clad layer for the indiumgallium nitride active layer 31. Similarly, the top contact layer is ap-type gallium nitride layer 36 as in FIGS. 10 and 11. As a clad andtransition layer, FIG. 12 includes the portion 40 that is progressivelycompositionally graded between n-type aluminum gallium nitride at theinterface with the InGaN active layer 31 and substantially entirelyn-type gallium nitride at the interface with the p-type gallium nitridelayer 36. As a result, the p-n junction 33 is again made as ahomojunction between the nGaN portion of the graded layer 40 and thep-GaN layer 36.

The thickness of the layer or layers between the active layer and thep-n junction affects the functionality of the device. Layers that aretoo thin fail to offer the appropriate confinement, while layers thatare too thick allow too much recombination to take place in the thicklayer rather than in the active layer as desired. Accordingly, withrespect to the embodiment depicted in FIG. 1, clad layer 12 should bebetween about 30 and 70 Å thick. With respect to the embodiment depictedin FIG. 3, clad layer 12 should be between about 20 and 50_Å thick, andlayer 19 should be between about 30 and 50 Å thick. The total thicknessof layers 12 and 19 should preferably be no more than about 100 Å. Withrespect to the efficiency of the devices, one goal is to minimize thenonradiative recombination current (J_(nr)) while maximizing theradiative combination current (J_(r)). In this regard, the structureshown in FIG. 8 has the greatest (i.e., least desirable) nonradiativerecombination current. The nonradiative recombination current of thedevice of FIG. 9 is somewhat less than that of FIG. 8, but still greaterthan the more favorably lower nonradiative recombination current ofFIGS. 10, 11 or 12.

In the drawings and the specification, typical embodiments of theinvention have been disclosed. Specific terms have been used only in ageneric and descriptive sense, and not for purposes of limitation. Thescope of the invention is set forth in the following claims.

1. A semiconductor structure for light emitting devices that can emit inthe red to ultraviolet portion of the electromagnetic spectrum, saidstructure comprising: an n-type single crystal silicon carbide substrateof a polytype selected from the group consisting of 3C, 4H, 6H, and 15R;a p-type layer formed of at least one Group III nitride selected fromthe group consisting of gallium nitride, indium nitride, andIn_(x)Ga_(1-x)N, where 0<x<1; an active layer ofAl_(x)In_(y)Ga_(1-x-y)N, where 0≦x<1 and 0≦y≦1 and (x+y)≦1, wherein saidactive layer is n-type and is positioned between said silicon carbidesubstrate and said p-type layer; a first n-type cladding layer ofAl_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1 and 0≦y≦1 and (x+y)≦1, wherein saidfirst n-type cladding layer is positioned between said silicon carbidesubstrate and said active layer; a second n-type cladding layer ofAl_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1 and 0≦y<1 and (x+y)≦1, wherein saidsecond n-type cladding layer is positioned between said active layer andsaid p-type layer; wherein said first and second n-type cladding layershave respective bandgaps that are each larger than the bandgap of saidactive layer.
 2. A semiconductor structure according to claim 1, whereinsaid first n-type cladding layer has a first surface and a secondsurface, said first surface of said first n-type cladding layer being incontact with said silicon carbide substrate, and said second surface ofsaid first n-type cladding layer being in contact with said activelayer, wherein the composition of said first n-type cladding layer isprogressively graded such that the crystal lattice at said first surfaceof said first n-type cladding layer more closely matches the crystallattice of said silicon carbide, and the crystal lattice at said secondsurface of said first n-type cladding layer more closely matches thecrystal lattice of said active layer.
 3. A semiconductor structureaccording to claim 1, wherein said second n-type cladding layer has afirst surface and a second surface, said first surface of said secondn-type cladding layer being in contact with said active layer, and saidsecond surface of said second n-type cladding layer being in contactwith said p-type layer, wherein the composition of said second n-typecladding layer is progressively graded such that the crystal lattice atsaid first surface of said second n-type cladding layer more closelymatches the crystal lattice of said active layer, and the crystallattice at said second surface of said second n-type cladding layer moreclosely matches the crystal lattice of said p-type layer.
 4. Asemiconductor structure according to claim 1, wherein said p-type layeris magnesium-doped gallium nitride.
 5. A semiconductor structureaccording to claim 4, wherein said second n-type cladding layer is thickenough to deter migration of magnesium from said p-type layer to saidactive layer, yet thin enough to facilitate recombination in the activelayer.
 6. A semiconductor structure according to claim 1, wherein saidp-type layer comprises a superlattice formed from alternating layers oftwo Group III nitride layers selected from the group consisting ofgallium nitride, indium nitride, and In_(x)Ga_(1-x)N, where 0<x<1.
 7. Asemiconductor structure according to claim 1, further comprising a thirdn-type layer of Al_(x)In_(y)Ga_(1-x-y)N, where 0≦x≦1 and 0≦y<1 and(x+y)≦1, wherein said third n-type layer is positioned between saidsecond n-type cladding layer and said p-type layer.
 8. A semiconductorstructure according to claim 7, wherein said third n-type layer has afirst surface and a second surface, said first surface of said thirdn-type layer being in contact with said p-type layer and said secondsurface of said third n-type layer being in contact with said secondn-type cladding layer.
 9. A semiconductor device according to claim 1,further comprising a conductive buffer layer consisting essentially ofaluminum gallium nitride having the formula Al_(x)Ga_(1-x-y)N, where0≦x≦1, said conductive buffer layer positioned between said siliconcarbide substrate and said first n-type cladding layer.
 10. Asemiconductor structure according to claim 9, further comprising ann-type transition layer of a Group III nitride, said transition layerbeing positioned between said conductive buffer layer and said firstn-type cladding layer, and having the same conductivity type as saidfirst n-type cladding layer.
 11. A semiconductor structure according toclaim 1, further comprising discrete crystal portions selected from thegroup consisting of gallium nitride and indium gallium nitride, saiddiscrete crystal portions positioned between said first n-type claddinglayer and said silicon carbide substrate, said discrete crystal portionsbeing present in an amount sufficient to reduce the barrier between saidfirst n-type cladding layer and said silicon carbide substrate, but lessthan an amount that would detrimentally affect the function of anyresulting light emitting device formed on said silicon carbidesubstrate.